In current processes of forming cavities for epitaxial growth of source/drain regions, non-vertical cavity sidewalls are formed, and conventional source/drain implantations result in non-uniform doping profiles. Consequently, a non-conformal junction is formed which in turn leads to threshold voltage non-uniformity along the fin height. During the device operation, the non-conformal junction will prevent the fin active region from full utilization, and it also degrades channel resistance and spreading resistance. Besides, the junction invasion at the fin tip worsens short channel effects.
FIG. 1A illustrates a desired cavity sidewall. FIG. 1B illustrates an implant profile after source/drain implantation, which includes a sloped sidewall (at 101). Adverting to FIG. 1C, after all thermal processes, the resulting dopant profile forms a gradient with a decreasing concentration from 103 to 113. As illustrated in FIG. 2A, conventional low energy and heavy dose source/drain implantation after epitaxial growth aimed for ohmic contact will introduce excessive dopant at the fin tip region 201. If a moderate energy source/drain implantation is employed, the middle to bottom effective gate length Leff is slightly reduced, but the junction overall remain sloped in FIG. 2B. A high energy source drain implantation still cannot straighten the junction and it will cause serious tailing, as illustrated at 205 in FIG. 2C.
The conventional extension implantation techniques cannot straighten the junction. As illustrated in FIG. 3A, for FinFETs with fins 301 having a pitch 303 between 20 and 40 nm, extension implantation 305 is tilted with respect to the fins 301 to cover the entire fin sidewall. However, as illustrated in FIG. 3B, the resultant implantation 307 is non-conformal and non-uniform, and it will also cause both fin damage and junction uniformity issues. Thus, conventional implantation before or right after source/drain epitaxial growth will cause a graded junction, undesired junction tailing, a non-conformal junction, and fin damage.
A need therefore exists for methodology enabling formation of both a conformal junction with a high epi surface dopant concentration in a FinFET and the resulting device.